Attaching a layer of viscoelastic material to component parts of a mechanical or electromechanical system may reduce unwanted noise and vibration, helping to diminish the propagation of structure-borne noise and the transmission of airborne noise. A two-layered or unconstrained type damping structure is made by providing a viscoelastic layer of rubber or synthetic resin on a metal plate. A three-layered or constrained type damping structure comprises a viscoelastic core sandwiched between a pair of metallic constraining layers. The ability of the damping structure to damp vibrations is known as its “loss factor”, with a higher loss factor indicating greater damping capability.
For constrained layer dampers (CLD), a force applied to the constraining layers drives the viscoelastic material into shear along the constraining layers, thereby converting a substantial amount of vibrational energy into heat. Increasing the shear within the damping structure, therefore, also increases the energy dissipating characteristics therein. It is thus desirable to provide a damping structure with increased shear to increase the loss factor. A method of making such a constrained layer viscoelastic laminate structure is disclosed in commonly assigned U.S. Pat. No. 6,202,462, to Hansen et al., issued Mar. 20, 2001, which is hereby incorporated by reference in its entirety.
Constrained layer dampers are sometimes used, for example, in the automotive industry for vehicle body panels as well as damping inserts for automobile brake systems. Traditionally, the viscoelastic core of the damping insert for automobile brake systems is made of a thermoplastic or a thermosetting material. Damping inserts with a thermoplastic-type viscoelastic core, such as thermoplastic pressure sensitive adhesives (PSA) and hot-melt-adhesive films, may encounter delaminating problems during harsh conditions, and from the high temperature and high pressure generated by many automotive brake systems. Comparatively, thermosetting-type adhesives, such as epoxy and phenolic resin, provide higher bonding strength, but may not offer sufficient damping capacity due to the high cross-linking density of the thermosetting materials.
The use of vulcanized rubber as the viscoelastic core for a CLD provides higher bonding strength than traditional thermoplastic-type viscoelastic core, and good sound and vibration damping characteristics. The higher bonding strength is needed for harsh application conditions (e.g., during stamping processes) and higher temperature applications (e.g., for brake shims, etc.).
A method of making a CLD with a vulcanized rubber viscoelastic core is disclosed in U.S. Pat. No. 5,213,879, to Niwa et al. (hereinafter “Niwa”), issued May 25, 1993, which is hereby incorporated by reference in its entirety. The Niwa patent relates to automotive brake inserts constructed by laminating a vulcanized rubber sheet onto a metallic constraint plate. Specifically, Niwa proposes to use a rigid polyamide adhesive film to bond a vulcanized rubber sheet onto iron plates. Unlike the present invention, Niwa's method vulcanizes the NBR rubber into a sheet, then piles the rubber sheet onto an iron plate with an epoxy primer treatment, places a second iron plate with an epoxy primer treatment on top of the pre-vulcanized rubber sheet, and subsequently laminates the structure in a single, discontinuous step using a hot press.
A method of making a CLD with a vulcanized rubber viscoelastic core is also disclosed in U.S. Pat. No. 5,853,070, to Josefsson (hereinafter “Josefsson”), issued Dec. 29, 1998, which is hereby incorporated by reference in its entirety. Josefsson discloses a method of making steel-rubber-steel laminate brake inserts. In the method of Josefsson, an uncured rubber film is applied between two layers of steel, and vulcanized in a lengthy, continuous process. It is the vulcanized rubber that acts as the bonding layer for Josefsson's brake insert, as well as the vibration damping viscoelastic core for the CLD.
However, vulcanizing rubber in a continuous process requires a large number of expensive ovens to maintain adequate throughput. Moreover, the use of thin calendered rubber sheets as taught by Josefsson requires expensive calendering equipment. In addition, applying the rubber sheet during the coil process requires use of an expensive carrier sheet. Also, in order to prevent separation of the steel constraining layers at high vulcanization temperatures, a special vulcanizing machine and expensive escort webs are needed to complete the process. Finally, the thickness of calendered rubber sheets is difficult to control, especially at low thicknesses—e.g., on the order of 0.10-0.15 millimeters or 4-6 mils.